Chemotherapy, especially alkylator chemotherapy, is one of the primary modes for the treatment of cancer; however, the response rate is often low. A major reason for poor prognosis in many patients is the development of drug resistance, resulting in tumor regrowth and eventual patient death. The drug resistance is due to the repair of DNA lesions, generated by alkylation of guanine and thymine residues by the alkylating agents, by the repair protein O6 alkyl-DNA alkyltrasferase (AGT). AGT repairs the lesions by transferring the alkyl groups from the modified DNA to a cysteine in its active site. This is a suicidal process as there is no mechanism to regenerate the protein and hence the resistance, or alternatively the therapeutic efficacy, depends on the amount of AGT content in the tumor. A means to noninvasively quantitate tumor AGT will go a long way in the planning of alkylator chemotherapy, monitoring the effect of drugs on AGT levels during the therapeutic course, and in the development of newer alkylator chemotherapeutic agents. Benzylguanine (BG) has been shown to be an excellent inactivator of AGT and there are several clinical trials in progress evaluating the use of BG to deplete AGT prior to chemotherapy. The long-term objective of this proposal is to develop radioiodine- or 18F-labeled BG analogues useful in the noninvasive mapping of AGT by scintigraphic imaging. BG containing fluorine at the 4-position of its benzyl group (4-FBG) has already been shown to be an excellent analogue of BG. Methods will be developed for the 18F-labeling of 4-FBG as well as radioiodinated BG derivatives. While it may be possible to demonstrate the proof of principle using these compounds, efforts will also be directed in the development of better analogues that are easier to prepare, more potent, and will have higher metabolic stability, and longer tumor retention. Among the compounds considered are the derivatives of 4-benzyloxy-2,6-diamino-5-nitro/nitroso-pyrimidine, and various BG derivatives with substituents at its C-8 and N-9 positions. Novel unlabeled compounds will be first evaluated for their AGT inactivating ability using Chinese hamster ovary (CHO) cells transfected with AGT. Labeled compounds will be evaluated for their binding with pure AGT and for their uptake and retention in AGT-containing cells. Their in vitro metabolism also will be studied. Correlativity of tumor AGT content and tumor uptake of labeled BG analogues will be Established. Primarily, TE-67 1 human medulloblastoma xenografts will be utilized for these studies. Tumor uptake as a function of drug concentration and time will be studied. Potential metabolites, especially in the tumor, will be determined. Again, the correlativity between tumor uptake and tumor AGT content will be investigated in xenograft models as well. In summary, this proposal seeks to determine whether the development of a suitable agent for the imaging of tumor AGT will facilitate chemotherapy planning, thereby improving the outcome of alkylator chemotherapy. Subsequently these tracers will be evaluated in athymic mice hosting xenografts. Primarily, TE-671 human medulloblastoma xenografts will be utilized for these studies. Tumor uptake as a function of drug concentration and time will be studied. Potential metabolites, especially in the tumor, will be determined. Again, the correlativity between tumor uptake and tumor AGT content will be investigated in xenograft models as well. In summary, this proposal seeks to determine whether the development of a suitable agent for the imaging of tumor AGT will facilitate chemotherapy planning, thereby improving the outcome of alkylator chemotherapy.